1. Field of the Invention
The present invention relates to an electrodeionization apparatus having a structure in which a plurality of cation-exchange membranes and a plurality of anion-exchange membranes are alternately arranged between a cathode and an anode in such a manner as to alternately form a plurality of diluting compartments and a plurality of concentrating compartments, particularly to an electrodeionization apparatus which is improved in spacers for making up the concentrating compartments so that the quality of the deionized water produced by the apparatus is improved.
2. Description of the Related Art
An electrodeionization apparatus has been widely used for producing the deionized water in various kinds of industry and research establishments including manufacturing plants of semiconductors and liquid crystal displays, the pharmaceutical manufacturing industry, the food processing industry, the electric power industry and the like, because the electrodeionization apparatus has a good feature that makes it possible to produce a deionized water with exceedingly high purity in completely continuous service without maintenance such as regeneration which is indispensable to the ion-exchange resin.
An electrodeionization apparatus has a structure in which a plurality of cation-exchange membranes and a plurality of anion-exchange membranes are alternately arranged between electrodes in such a manner as to alternately form diluting compartments and concentrating compartments and the diluting compartments are filled with an ion exchanger. Voltage is applied between the cathode and the anode of the electrodeionization apparatus, water to be treated is introduced into the diluting compartments and concentrated water is introduced into the concentrating compartments, so that impurity ions permeate the membrane from the water to be treated to the concentrated water, thereby producing deionized water.
FIG. 2 is an exploded view showing the structure of the electrodeionization apparatus.
The electrodeionization apparatus includes a cathode end plate 1, a cathode 2 extending along the end plate 1, a cathode spacer 3 extending along the outer periphery of the cathode 2 which are superposed in this order. Further, a cation-exchange membrane 4, a frame 5 for defining a diluting compartment, an anion-exchange membrane 6, and a gasket 7 for defining a concentrating compartment are superposed on the cathode spacer 3 in this order. The cation-exchange membrane 4, the frame 5 for defining a diluting compartment, the anion-exchange membrane 6, the gasket 7 for defining a concentrating compartment compose one unit. The apparatus is composed of a plurality of such units superposed together. That is, membranes 4, frames 5, membranes 6, and gaskets 7 are repeatedly superposed one unit over the other unit. An anode 9 is superposed between the last anion-exchange membrane 6 and an anode spacer 8. An anode end plate 10 is superposed on the anodic electrode 9. The apparatus is tightened by bolts or the like.
The space defined by the inner surface of the frame 5 is the diluting compartment in which an ion exchanger 5R such as ion-exchange resin is filled. The space defined by the inner surface of the gasket 7 is a concentrating compartment. A mesh 7M is arranged inside the gasket 7 of a spacer for defining the concentrating compartment.
A direct electric current is supplied to pass between the anode 9 and the cathode 2, raw water to be treated is fed to the diluting compartment through a raw water inlet line 11, and concentrated water is fed to the concentrating compartment through a concentrated water inlet line 12. The raw water fed to the diluting compartment flows through a layer filled with the ion-exchange resin whereby impurity ion in the raw water is removed so as to make the raw water to deionized water which flows out through a deionized water outlet line 13.
The concentrated water fed to the concentrating compartment captures impurity ions which pass through the ion exchange membranes 4, 6 while flowing down through the concentrating compartment, and flows out from a concentrated water outlet line 14. Electrode water is passed within electrode compartments through introducing lines 15, 16 and discharging lines 17, 18, respectively.
FIG. 1a is a perspective view of showing the form of the spacer for defining the concentrating compartments, and FIG. 1b is a lateral view thereof. As shown in these figures, the spacer for defining the concentrating compartments has an integral structure consisting of the parallel frame-shaped gaskets 7A, 7B and the mesh 7M arranged between these gaskets. A reference numeral 7a denotes an inlet for the concentrated water, and a numeral 7b denotes an outlet for the concentrated water. Numerals 7c and 7d denote flow inlets for the raw water and deionized water respectively.
PCT-Japanese phase H6-506867 describes that an apparatus having spacers defining the concentrating compartments is improved in the efficiency of removing silica as the velocity of water flow in the concentrating compartment is increased.
While the electrodeionization apparatus efficiently provides the deionized water with exceedingly high purity in completely continuous service without maintenance including regeneration, there are further demands towards improvement in the purity of the produced water.
It is an object of the present invention to provide an electrodeionization apparatus which is improved in concentration polarization of ingredients including ions especially in the concentrating compartment so as to obtain the quality of the produced water with high purity.
An electrodeionization apparatus of the present invention is provided with concentrating compartments, each of which is composed of a mesh and a frame-shaped gasket superposed on periphery of the mesh, and diluting compartments defined by a plurality of ion exchange membranes between a cathode and an anode, wherein the mesh has a thickness in the range of 0.2 to 0.5 mm and the gasket has a thickness of equal to or less than 0.1 mm.
The inventors of the present invention found matters below-described by their elaborate study for the purpose of improving the quality of the produced water.
That is, in the electrodeionization apparatus, ingredients including ions removed from a diluting compartment travel into a concentrating compartment through an ion exchange membrane. On this occasion, in case the ingredients including the ions move slowly on the surface facing the concentrating compartment, the efficiency of removing the ingredients in the electrodeionization apparatus is deteriorated. Therefore, promotion of turbulence at a mesh portion so as to speed up the movement of the ingredients including the ions on the surface of the membrane of the concentrating compartment is important with regard to function of a spacer of the concentrating compartment.
Accordingly, the inventors further found that to thin the mesh portion of the spacer for defining the concentrating compartment to a thickness of 0.2 to 0.5 mm and the gasket portion thereof to a thickness of equal to or less than 0.1 mm improves adhesion of the mesh and the ion exchange membrane to prompt the turbulence, so that the produced water with high purity can be obtained to complete the present invention.
The spacer for defining the concentrating compartment of the present invention improves the efficiency of removing the ions to be ascribed to the promotion of occurrence of the turbulence at the mesh portion to accelerate the movement of the ions on the surface of the membrane facing the concentrating compartment. Further, the concentrating compartment is made thinner, so that the electrical resistance between electrodes can be lowered to improve the electrical efficiency.